That achievement is a 4 part achievement -- after reaching the 250 population, you must next have $20,000 in your Swiss Bank Account for the current mission, then a population happiness of 60, and finally, pay $20,000. After you pay, the palace goes through a quick auto-upgrade, after which the achievement pops.

Just realized that yesterday and unlocked it, just have the secret achievements left to get myself after mission 3 so it is an easy 250gs.